In industrial production, a colloidal solution is frequently used. Due to the fact that a density of the colloidal solution is relatively large and the flowability is relatively poor, it is prone to produce foams that may be not easy to be removed from the colloidal solution. These foams may have an adverse impact on the subsequent processes. For example, in process of manufacturing a flexible substrate of a flexible display device, one of the important components of the flexible substrate is polyimides (PI) film. In order to obtain the PI film, it may be necessary to coat a PI colloidal solution onto glass by means of a coating process and then to perform high-temperature curing treatment on the glass. However, the PI colloidal solution may often contain a lot of foams. In order to guarantee the quality of the PI film and avoid the impact of deteriorated PI film on the flexible substrate, the foams therein need to be removed before the PI colloidal solution is coated onto the glass.